usb: phy: am335x-control: make it compile with

Randy reported this
|drivers/usb/phy/phy-am335x-control.c:45:3: error: implicit declaration
|of function '__WARN' [-Werror=implicit-function-declaration]

and left it as an excercice to figure out that this happens only with
CONFIG_BUG=n. As a fix I replace it with WARN_ON(). And there is a space
before return so fix this, too.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>